Changed some of the wording in units docs to clarify what "unit formats" is about
added more subsections and generified base formats.rst title
Changed format.rst section title to @mdboom's suggestion

[ci skip]
Changed section title form "Units" to "Units and Quantities"

[ci skip]
Convert to degrees only once in _validate_angle
Convert to degrees only once in _wrap_angle
Skip the error checking in the CompositeUnit constructor when we know
it's redundant.  Return `self` when scaling by 1.
Don't invoke the unit parser when handling the Quantity convenience
conversion methods.
Use __quantity_view__ rather than __quantity_instance__ inside of
`Quantity.to`, and with the subsequent bugfixes to support that.
Further optimize angle bounds checking routines by converting as little as possible
Progress
Remove unnecessary Unit call; normalise equivalencies when it is needed
Replace (self/other)._dimensionless_constant() by fast _to
Remove _dimensionless_constant method, as it is no longer needed
Ensure Unit(<unit>) returns immediately
Shortcut array in _condition_arg
Short circuit when converting unit to same unit
Swap order in trying equivalencies; funit and tunit should decide
cleanup of condition_arg; is_effectively_unity speedup
Use _to everywhere, replacing is_equivalent/to combinations
Make is_effectively_unity slightly laxer so it can replace all_close